Why is the native's opinion of kurtz surprising?

Kurtz has set himself as a sort of white messiah to some of the tribes deep in the Congo. They like him because he brings certain tribes power. Perhaps Kurtz has a European penchant for organizing indigenous peoples for his own gain but certainly some tribes like what he is doing. Kurtz however is also a prisoner of his own making. His unyielding surrender the jungle means the "powers of darkness have claimed him for their own."  Kurtz can never walk away.